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'86 B2 2.9 miss at ~3200 RPM


G8orFord

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Ive had carb base gaskets not leak untill up in the RPM's....not saying thats the case here but it might be worth a look.
Oh, I know anything is possible at this point, but it does have new gaskets on the plenum of upper intake. I haven't had the TB off or the lower intake off, but I'm fairly certain they're good. Something to check though.
 


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So I went back in the thread... and see you put a new TPS on it. Chances of both being bad causing the same issue is slim. But did you set the voltage on the new TPS?

Could just be spitting in the wind but I think I would reset the idle speed using this procedure from a sticky in the 2.9L forum...


Once idle speed is set... set the TPS voltage. I "think" the spec is .96 - .98 volts at base idle. Should also be about 4.5 volts at WOT. To check voltage on the TPS... you pierce the insulation on the TPS signal wire using a straight pin. Then use a DMM to check voltage on that circuit with key ON. To test the full sweep of the TPS... I still have an analog volt meter. I use that to watch for a smooth increase in voltage all the way up to WOT. A DMM in that case is just too jumpy to see what's really going on. This is the schematic for my 86... not sure if it's the same.

20240310_215546.jpg


Like said... I would do this just to ensure you have the correct baseline... then you can just cross it off the list.
 

G8orFord

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Just the right size to touch the ground.
So I went back in the thread... and see you put a new TPS on it. Chances of both being bad causing the same issue is slim. But did you set the voltage on the new TPS?

Could just be spitting in the wind but I think I would reset the idle speed using this procedure from a sticky in the 2.9L forum...


Once idle speed is set... set the TPS voltage. I "think" the spec is .96 - .98 volts at base idle. Should also be about 4.5 volts at WOT. To check voltage on the TPS... you pierce the insulation on the TPS signal wire using a straight pin. Then use a DMM to check voltage on that circuit with key ON. To test the full sweep of the TPS... I still have an analog volt meter. I use that to watch for a smooth increase in voltage all the way up to WOT. A DMM in that case is just too jumpy to see what's really going on. This is the schematic for my 86... not sure if it's the same.

View attachment 107344

Like said... I would do this just to ensure you have the correct baseline... then you can just cross it off the list.
The "new" TPS was out of range, but the old FoMoCo one checked fine and is back in place. I did discover a broken gasket on the idle air motor which was causing a slight vacuum leak, but fixing that didn't change the miss. I'm running out of ideas.
 

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The "new" TPS was out of range, but the old FoMoCo one checked fine and is back in place. I did discover a broken gasket on the idle air motor which was causing a slight vacuum leak, but fixing that didn't change the miss. I'm running out of ideas.

You know anyone with an oscilloscope?

That would let you see the secondary ignition. If it were ignition related... you'd see it.

It still could be... but you've changed all (?) The secondary ignition parts. Plugs... wires... cap... rotor and even a coil... still the same issue. For the hell of it... when dark... spritz the ignition down with water and have a look. Take it up to where is starts missing and look for spark leakage.

It appears to come in right during the point between low and high speed fuel strategy.

Does it have an EGR?

Did you replace the MAP?
 

G8orFord

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You know anyone with an oscilloscope?

That would let you see the secondary ignition. If it were ignition related... you'd see it.

It still could be... but you've changed all (?) The secondary ignition parts. Plugs... wires... cap... rotor and even a coil... still the same issue. For the hell of it... when dark... spritz the ignition down with water and have a look. Take it up to where is starts missing and look for spark leakage.

It appears to come in right during the point between low and high speed fuel strategy.

Does it have an EGR?

Did you replace the MAP?
I've blocked the EGR. No change. I've Replaced the EGR. No change. I've replaced the MAP (pretty sure the one I replaced was new). No change. There is vacuum to the MAP at idle.
 

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I've blocked the EGR. No change. I've Replaced the EGR. No change. I've replaced the MAP (pretty sure the one I replaced was new). No change. There is vacuum to the MAP at idle.
You dont have any valvetrain noise do you?

Wondering if maybe its a lifter oiling issue? I really have no other guesses at this point and am totally pissing in the wind.
 

G8orFord

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Maybe a little lifter chatter after it's sat a few days, but nothing major.
Here's a video of what is doing. At this point, I'm thinking broken valve spring or partially wiped cam lobe.
 

G8orFord

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Did a cylinder leak down test today. #4 is only at 40% and #6 was at 70%. The others were all 90+. Wouldn't have expected that after the compression test was reasonable, but those were the two lowest cylinders on that test too.

How much trouble is swapping in a 4.0? If I'm going to rebuild a motor, I might as well get something out of it.
 
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alwaysFlOoReD

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Where was the leakage?
 

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G8orFord

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Where was the leakage?
It was hard to tell for sure, I believe most of it was going out the exhaust. A little Marvel Mystery in #4 did bring it up to about 50%. Didn't bother on #6.
 

G8orFord

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Just the right size to touch the ground.
#8??

Generally speaking if lower end is under 400k miles just new/rebuilt heads will restore engine

2.9l to 4.0l swaps are well documented in this tech section: https://www.therangerstation.com/tech/engines-fuel-systems/

Scroll down to V6 swaps

"How much trouble......" is a relative term :)
Typo #6.

Heads might fix it, but if I'm going that far, I'm going to check the rest of it too. Do you think these numbers could cause the missing problem? I'm not 100% convinced.

Yeah I poked around in the 4.0 swap section a little. Not really interested in changing the dash or doing a lot of rewiring.
 

G8orFord

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Just the right size to touch the ground.
To finish my previous post, I had all I could stand for the day, so called it. It'll probably be next weekend before I put it back together. Though I don't expect any difference, I did re-adjust the valves on #4 since there was some slop. That wouldn't have any effect on the leak down test though.
 

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